Lithium-ion batteries (LIBs) are presently considered as the key technology in electromobility. They must be highly optimized with regard to their cost, weight, energy density, service life, safety, and charge life.
By use of innovative electrode materials it is possible to increase the energy density of lithium-ion batteries (135 Wh/kg (2013) to 280 Wh/kg (2018)), and thus to significantly increase the cruising range of electric vehicles (from 190 km to 500 km). Silicon is a promising active material in this regard. Silicon has a capacity ten times that of commercially used graphite, and has a similarly low lithiation potential (0.5 V vs. lithium-lithium+). Since silicon is the second most common material in the earth's crust and thus has low production costs, and the material is safe to handle and is nontoxic, it is attractive from an industrial standpoint.
It is known that during the first cyclings of a battery, a solid electrolyte interface (SEI), i.e., a boundary layer between the negative electrode and the electrolyte, is formed due to reductive decomposition of various electrolyte components such as solvents, additives, and impurities; this solid electrolyte interface is not thermodynamically or electrochemically stable at the voltages achieved.
The formation of the SEI is on the one hand essential for the functionality and the service life of lithium-ion batteries, since in the ideal case it has good ionic conductivity and at the same time has an electrically insulating effect. Due to its kinetically limiting effect, it largely suppresses further decomposition of the electrolyte and counteracts further capacity losses. On the other hand, the SEI protects the structure of the active material (graphite) from exfoliation, and thus protects the cell from significant capacity losses.
However, during the formation of the SEI, irreversible capacity loss which is attributed to the formation also always occurs.
In the case of commercial graphite electrodes, the irreversible capacity loss due to the SEI formation is very low, at approximately 2 to 5%, in relation to the silicon-containing negative electrodes, at 20 to 80%.
For the silicon-containing negative electrodes, which are particularly susceptible to irreversible capacity loss, a distinction must be made between two different types of irreversible capacity loss. In addition to the capacity loss during the initial formation, i.e., an initial capacity loss, capacity loss due to “breathing” during the cycling also occurs.
Therefore, a fundamental challenge to the commercial application of silicon-containing negative electrodes is the enormous change in volume, i.e., the breathing, of the material during the lithiation and delithiation processes (Li4Si15: 280%-300%, compared to LiC6: 10-11%). The breathing of the silicon-containing negative electrode results in pulverization of the particles, and thus, additional problems. In particular, it has catastrophic effects on the preservation of the electrode architecture, which occur in particular with high surface loading. This causes contact losses within the electrode and between the electrode and the current collector, and is reflected in impairment of the electrical conductivity. In addition, it results in constant fracturing and growth of the SEI. This in turn results in continuous Li ion consumption and increasing internal resistance in the cell, and thus a lower coulombic efficiency (CE) and inadequate cycle stability.
The initial capacity loss is greatly dependent on which silicon species is used. The species may differ, among other criteria, in the type of material (silicon alloy, silicon composite, silicon oxide, coated silicon), in the shape, in the crystallinity (amorphous, crystalline, or polycrystalline), and in the particle size (nano range, micro range, size distribution), and thus in the SEI surface. In particular particles in the nano range have an active surface and thus a higher initial capacity loss. In this case, the SEI is particularly thin due to a constant current that is better distributed over the active surface.
DE 11 2015 000 403 T5 discloses a method for improving the performance of a silicon-based negative electrode by prelithiation of the silicon-based negative electrode in an electrolyte containing dimethoxyethane, fluoroethylene carbonate, and a lithium salt. In particular, the preformation of an SEI on the silicon-based negative electrode in the prelithiation electrolyte by applying a voltage for a period of 1 to 100 hours is disclosed.
U.S. Pat. No. 9,293,770 B2 discloses a method for coating a silicon-based negative electrode with a graphene oxide layer. According to this method, the graphene oxide layer is applied to a silicon-based negative electrode by electrophoresis. Current is applied at a voltage of 0.5 V to 2.0 V for at least 0.5 min to 30 min.
U.S. Pat. No. 8,801,810 B1 discloses a method for manufacturing a lithium-ion battery, wherein the lithium-ion battery is first charged up to a certain voltage at a constant C-rate, then held at constant voltage until the current intensity reaches a specified value. The lithium-ion battery is subsequently stored for a given period of time, with a possible storage time of 0 to 12, 24, 48, and 72 hours. The lithium-ion battery is subsequently discharged to a certain voltage at a constant C-rate, and the cell is then further discharged at constant voltage until a specified capacity value is reached.